Since a fuel cell utilizes energy generated during a binding reaction between hydrogen and oxygen, fuel cells contributes to a next-generation power generation system whose introduction and widespread use are expected from the viewpoint of energy-saving and environmental measures. Examples of the fuel cells include a solid electrolyte fuel cell, a molten carbonate fuel cell, a phosphoric acid fuel cell, and a solid polymer electrolyte fuel cell.
Of those, the solid polymer electrolyte fuel cell has high output density, is capable of being reduced in size, operates in lower temperature than other types of fuel cells, and is easily started and stopped. From those advantages, use of solid polymer electrolyte fuel cells for small-sized cogeneration for automobiles and homes has been expected, and has recently been drawing attention particularly.
FIG. 1 is a diagram showing a structure of a solid polymer electrolyte fuel cell (hereinafter, may also be simply referred to as “fuel cell”). FIG. 1(a) is an exploded perspective view of a single cell included in the fuel cell, and FIG. 1(b) is a perspective view of an entire fuel cell made by combining multiple single cells.
As shown in FIG. 1, a fuel cell 1 is a stack of single cells. As shown in FIG. 1(a), viewing a single cell, an anode-side gas diffusion layer (also called “fuel electrode film”, hereinafter, referred to as “anode”) 3 is stacked on one side of a solid polymer electrolyte membrane 2, and a cathode-side gas diffusion layer (also called “oxidizing agent electrode film”, hereinafter, referred to as “cathode”) 4 is stacked on the other side of the solid polymer electrolyte membrane 2; and on the both sides thereof, separators (bipolar plates) 5a and 5b are stacked, respectively.
Some fuel cells have separators having a gas flow channel for cooling water to pass through, each separator being placed between two adjacent single cells or every several single cells. The present invention also targets at such a water-cooled fuel cell separator.
As a solid polymer electrolyte membrane (hereinafter, simply referred to as “electrolyte membrane”) 2, mainly used is a fluorine-based proton conductive membrane having a hydrogen ion (proton)-exchange group.
As each of the anode 3 and the cathode 4, mainly used is a carbon sheet (or carbon paper having a thickness smaller than the thickness of the carbon sheet, or a carbon cloth having a thickness smaller than the thickness of the carbon paper) obtained by rendering conductive carbon fibers into a sheet shape. There are cases where the anode 3 and the cathode 4 are each provided with a catalyst layer including a particle-shaped platinum catalyst, graphite powder, and, as necessary, a fluorine resin having a hydrogen ion (proton)-exchange group. In those cases, fuel gas or oxidizing gas comes into contact with the catalyst layer and the reaction is promoted.
Regarding the separator 5a, on the surface of the side of the anode 3, a flow path 6a having a groove shape is formed. Fuel gas (hydrogen or hydrogen-containing gas) A flows through the flow path 6a, and the anode 3 is supplied with hydrogen. Regarding the separator 5b, on the surface of the side of the cathode 4, a flow path 6b having a groove shape is formed. Oxidizing gas B such as air flows through the flow path 6b, and the cathode 4 is supplied with oxygen. Supplying of those gases causes an electrochemical reaction to occur and direct-current power to be generated.
Main functions that a separator of a solid polymer electrolyte fuel cell is demanded to have are as follows.
(1) A function as a “flow path” for supplying uniformly a surface of the cell with fuel gas or oxidizing gas.
(2) A function as a “flow path” for efficiently discharging water generated at the cathode side outside the system from the fuel cell together with air generated after reaction and a carrier gas such as oxygen.
(3) A function to be a passage for electricity by being in contact with the electrode films (anode 3 and cathode 4) and to be an electrical “connector” between two adjacent single cells.
(4) A function as a “partition wall” between, of adjacent cells, an anode chamber of one cell and a cathode chamber of the other cell.
(5) In a water-cooled fuel cell, a function as a “partition wall” between a cooling water flow path and an adjacent cell.
It is necessary that a material of a base material of a separator for a solid polymer electrolyte fuel cell (hereinafter, simply referred to as “separator”) be able to achieve those functions. The materials of a base material is roughly classified into metal-based materials and carbon-based materials. Using a carbon-based material, there is an advantage in that a lightweight separator can be obtained, but there are problems that the carbon-based material has gas permeability (the function as a partition wall is limited) and that the mechanical strength is low.
Examples of the metal-based materials include titanium, stainless steel, and carbon steel. The separator made of those metal-based materials is manufactured by press working and the like. The metal-based materials have, as characteristics unique to metals, advantages that processability is excellent, the thickness of the separator can be decreased, and that the weight of the separator is reduced, but has a risk that the electrical conductivity may decrease due to oxidation of the metal surface. Accordingly, there arises a problem that contact resistance between a separator made of a metal-based material and a gas diffusion layer may increase. In regard to this problem, the following measures are proposed.
Patent Literature 1 proposes a separator in which a surface of a metal member is plated with gold. Patent Literature 2 proposes a separator in which a noble metal thin film layer is formed on a surface of a metal base material.
The separators of Patent Literatures 1 and 2 each include a large amount of a noble metal. Accordingly, a separator in which an amount of gold included is reduced by performing plating with a noble metal partially is proposed. For example, Patent Literature 3 discloses a metal separator including a surface having corrosion resistance and a conductive inclusion shown above the surface, wherein an area other than an area in which the conductive inclusion is shown is covered with gold. Patent Literature 4 discloses a structure (separator) including a gold-plated portion and a non-plated portion on a surface of a titanium base material.
The following are separators which do not include gold. Patent Literature 5 proposes a titanium alloy whose increase in contact resistance is suppressed by pickling a titanium alloy containing one or more platinum group elements and concentrating the platinum group elements on a surface. Patent Literature 6 proposes a titanium separator in which platinum group element(s) is(/are) concentrated on a surface through pickling, and then, in order to improve adhesion between the platinum group element(s) concentrated on the surface and a matrix, heat treatment is performed in a low oxygen concentration atmosphere.
The following are a separator and an electrode, in which a conductive substance is dispersed (scattered) on a surface. Patent Literature 7 discloses a separator in which a metal coating film containing conductive ceramics is formed on a surface. The conductive ceramics are dispersed in the metal coating film. Patent Literature 8 discloses an electrode for electrolysis including a surface layer made of a metal oxide film, wherein a layer immediately under the surface layer includes noble metal(s), and, at an outer layer portion, the noble metal(s) is(/are) precipitated and dispersed in grain boundaries of the metal.
Patent Literature 9 discloses a separator obtained by forming a concave flow path on a titanium substrate, then forming a plated layer made of noble metal(s) such as Au and/or Pt on a substrate, and further performing heat treatment at 300 to 800° C. Patent Literature 10 discloses a corrosion-resistant conductive covering material in which a platinum group metal-plated layer is formed on an outer layer of a metal substrate, and two intermediate layers are formed between the metal substrate and the plated layer, the two intermediate layers including, from the side of the metal substrate, a layer (A): a thin film made of oxides of metals in the fourth group and the fifth group; and a layer (B): a thin film containing platinum group metal(s) or the oxide(s) thereof.